Primary lung abscess is a common clinical entity. It is an example of mixed infection due to aerobic and anaerobic bacteria. Aspiration is the most common cause of lung abscess. It is therefore not surprising that such infections are polymicrobial, generally consisting of anaerobic flora of the mouth. At present the role of individual organisms(anaerobes in particular) in such infections is unclear. We intend to develop and animal model and study thevarious aspects of lung abscess. The preliminary plan is inject pure cultures of selective strains of aerobes or anaerobes transtracheally into anaesthetized rabbits and study the progress of lung infection by serial roentgenograms. They are sacrificed at the end of two weeks or sooner if there is evidence of lung abscess. During this study we intend to inject pure and well characterized cultures of aerobic ad anaerobic bacteria to determine if they, by themselves, can produce lung abscess. We then plan to inoculate various bacterial combinations (aerobe-anaerobe and anaerobe-anaerobe). The purpose as mentioned is to study if bacterial synergism plays a role in the development of lung abscess. If successful in developing an animal model forlung abscess, we intend to evaluate the effect of selective elimination of aerobe-anaerobe mixed infection; antibiotic therapy can be directed at either the aerobe, anaerobe, or both. It will then be known which members of a mixed infection it is necessary to eradicate to effect cure of the lung abscess, aerobes or anaerobes by antibiotic therapy. This may provide clues to chose proper antibiotics in the treatment of lung abscess. Furture applications of this animal model are discussed.o